Tv shutter priority?

MillerPhotography

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Hi all, I've bought myself my first DSLR a Pentax K-R which is providing me a great deal of enjoyment as I try new things out. One of the features is exposing a shot over a period of time (which I'm sure all you non-beginners will understand). I have used this to good effect with a shutter time of a second or less but when I attempt anything more when pointing it at the sky example to capture cloud movement the photo is over-exposed (virtually White). I've dialled down exposure to under expose to no use. I understand in low light conditions the shutter stays open longer under normal circumstances to get the most available light, so is there too much light when keeping the shutter open on bright days?
 

analog.universe

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Yes, in order to compensate for adjustments in shutter speed, Tv mode adjusts the aperture automatically. Most lenses only stop down to f/16 or f/22, so once you've hit that limit, there's nothing else the camera can do, and the shot overexposes. You can get a neutral density filter to attach to the lens, which will block some portion of the light entering. It's essentially just a piece of grey glass, available in various step-down amounts. So, for example if you have a shot at f/22 exposed for 1 second, you could use a 4-stop neutral density filter and expose for 16 seconds instead.
 

Big Mike

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There are three things that affect your exposure. The length the time (shutter speed), the size of the lens aperture (F number), and the ISO.
Each one of those things can be used to control your exposure.

So if you want to use a long exposure time, you may have to compensate for that with a small aperture and/or a low ISO.

It sounds like the trouble that you're having, is that at the shutter speed you're choosing...the lens can't make it's aperture any smaller and your ISO can't go any lower, so you end up with an over exposed photo.
Here's a question for you...when you are in Tv mode and you choose a long shutter speed in those daylight conditions...does the aperture value (F number) flash in your viewfinder? If so, that is the camera telling you that it's reached it's limit and it can't go any smaller (or bigger, if you're on the other end). So that is your cue to know that you won't get the exposure you need, at the settings you want.

So what do you need to do? In this case, it's easy. You get yourself a neutral density (ND) filter. The filter blocks light, which allows you to get less exposure at the same settings. In other words, you can get proper exposure with a longer shutter speed than you could without the filter. There are many strength's of ND filters...you can stack them to block more light...or you could get a variable ND filter.

Also, if you're shooting the sky, I'd recommend a circular polarizing filter. It can help to make your skies look darker, while leaving the clouds bright. But it also blocks some light from the whole scene, allowing for a longer shutter speed.
 

tirediron

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Exactly correct. Making long exposures on bright days can be a challenge. You need to keep your ISO as low as possible, and use a small aperture (larger f #). Additionally, you may need/want to use a product called a 'neutral density filter' which is a filter that blocks out some of the light. I used three stops of ND filter to shoot this image with a shutter speed of 3 seconds on a bright sunny day. Without them, there would have been no way to acheive such a long shutter speed; you'll notice from the EXIF data that I'm stopped down to F22.
 
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MillerPhotography

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Wow I'm impressed by the speed of reply and depth of knowledge, and responses in simple English which is appreciated to a beginner. Big Mike - yes I believe the camera was telling me I was doing something stupid by flashing at me!.Thanks for your answers. I'll post some pics for C&C laterCheers
 

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